Download Beroual, A., M. Zahn, A. Badent, K. Kist, A.J. Schwabe, H. Yamashita, K. Yamazawa, M. Danikas, W.G. Chadband, and Y. Torshin, Propagation and Structure of Streamers in Liquid Dielectrics, IEEE Electrical Insulation Magazine, Vol. 14, No. 2, pp. 6-17, March-April 1998

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Propagation and Structure of
Streamers in liquid Diele
Key Words: Streamer pattern, propagation mode and velocity, pre-breakdown mechanisms
trary to tlie discharges in gases, the terin “streamer” in liquid
dielectrics also includes other structures, such as monochannel patterns.
These streamers are characterized by different structures
according to the experimental conditions. They produce
A. BEROUAL,
M. ZAHN,
A. BADENT,
IC. KIST, typical shapes of current transients or emitted light signals.
A.J. SCHWABE,
H. YAMASHITA,
IC. YAMAZAWA,They are accompanied by shock waves and their conductivM. DANII~S,
W.G. CHADBAND,
AND Y. TORSHINity depends on the mechanisms involved in their propagation. The streamer stops when the electric field becomes too
small, producing a string of micro-bubbles that dissolve in
the liquid. Note that Kerr effect measurements of the electric
T h e molecular structure
field found that streamers were highly conducting, with a
significant effect on streamer
voltage drop across the streamer that was < 10% of the total
voltage across the electrodes [I].
pro p a p t io n. T h e m a in para m e t er
Streamers are generally classified as slow and “bushy” for
affecting propagation is t h e
streamers emanating from the negative electrode, or fast and
“filamentary” for streamers emanating from the positive
electronic affinity of t h e liquid
electrode. Positive streamers are often (about ten times)
molecules.
faster than negative streamers, although transformer oil is an
exception, with positive and negative streamer velocities in
the same range. According to recent work, streamers can be
classified in different modes (l”,2lId, 31d, and even 4‘”) deINTRODUCTION
pending on their velocity and the polarity of the voltage [l].
he study of pre-breakdown and breakdown phenomena
Each propagation mode corresponds to a given inception
in liquid dielectrics has been the subject of many reelectric field. Therefore, the electrode geometry and the amsearch investigations during the past three decades. Our
plitude of the applied voltage have a significant influence on
understaiidiiig has been greatly improved by tlie use of fast
the structure, the velocity, and the mode of propagation of
digitizing oscilloscopes for voltage and current measurethe streamers. However, such a classification versus the poments; fast optical detecting systems for measurements of
larity is inconclusive when considering liquids of specific
density gradients using shadowgraph and Schlieren photogmolecular structure or containing selective additives.
raphy, coupled with an image converter camera; electric
The characteristics (structure, velocity, current.. ) of these
field distributions using electric field induced birefringence
streamers depend on the cheinical coinposition and physical
of the Kerr effect; and optical spectroscopy of emitted light.
properties of the liquid (pure or containing a small concen‘These methods have revealed a lot of information about the
tration of selected additives); pressure and temperature; the
processes occurring in the electrical breakdown of a dielecelectrode geometry; the voltage magnitude, polarity, and
tric liquid. Thus, it is possible to follow the different stages
shape; and contaminants of air, moisture, particles, and
leading to breakdown and to establish that the breakdown of
other trace impurities.
liquids is generally preceded by some events called “streamGenerally, ineasureinents use contoured parallel plane
ers,” where the optical refractive index is different from that
electrodes for uniform electric field studies. Rod-plane and
of the surrounding liquid and the structure is similar to that
point-plane electrodes are used to localize the discharge iniobserved in the breakdowns of gas and solid insulation. Contiation near the point using the field enhancement near the
by
The Liquid Dielectrics Committee International
Study Group, IEEE Dielectrics and Electrical
Insulation Society
has a
T
6
0883-7554/98/$10.000
1998
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IEEE Electrical Insulation Magazine
point to initiate breakdown at modest voltages. The use
of a point-to-point electrode geometry permits simultaneous observation of positive and negative streamers.
Typical voltage waveforms are standard lightning impulses (1.2/50 ps), impulses with short risetimes
(<200ns), rectangular pulsed voltages, decaying oscillating waveforms ( < l o 0 kHz-1 MHz), and power frequency ac voltages (50 or 60 Hz).
Our purpose is to present a critical review of the current
understanding of streamer propagation in liquids in order to
help define the direction of future research.
POLARITY
EFFECT
ON STRUCTURE
AND VELOCITY
STREAMERS
OF
The polarity effect in insulating oil in inhomogeneous
electric fields manifests differences in the generation, structure, and propagation velocity of negative and positive
streamers. Moreover, both classes of streamers pass through
several typical propagation stages (or modes) on their way
towards the counter electrode. Due to the different propagation velocities and patterns, the positive streamer can be classified in three consecutive stages: primary, secondary, and
tertiary streamers. In contrast, the negative streamer shows
only two propagation stages [2]. The appearance of each
structure depends on the field inception.
Positive Streamers
When increasing the voltage, one can first observe the
formation of positive primary streamers (PPS, or 1st mode)
exhibiting an intensified branching. The streamer appears in
an umbrella-like structure propagating with a constant velocity ranging from 2 to 3 km/s. The inception field of the
PPS was determined to be EI,pr+>2.0 MV/cm. Just before
contacting the opposite electrode, very fast events bridge the
remaining gap and initiate breakdown.
Higher voltage levels lead t o a reduced time to breakdown and to the formation of a positive secondary
streamer (PSS, or 2nd mode) as an extension of the primary structure area with the highest branching. The inception field of a PSS refers t o specific field strengths
EI,sec+> 1 2 MV/cm. Secondary streamers propagate with
a significant higher velocity that can reach 3 2 km/s. They
have a complex spatial structure in the form of a bright
main channel with a radius of the order 60-90pm, bright
lateral stems with a top zone of ionization in the shape of
brush-type structures with a 0.7-1.2 mm radius, consisting of not less than 20 weak luminous streamers with a radius of 3-6 p m (Fig. 1) [ 3 ] .At the streamer fronts, the
shock waves are recorded.
Like slow positive streamers, fast positive streamers extend by jumps through a zone of ionization, accompanied by
a streamer flash and by an increase of the ionization current
at the moment of flash up to 0.05-0.5 A. The frequency of
flashes increases with the average field E, (E, is the ratio between the applied voltage to the electrode gap). For E, = 55
kV/cm, this frequency is about 1 MHz and the minimum
March/April 1998 -Vol.
Fig. 1 Propagationof fast positive streamer in transformer oil in a point-plane electrode
geometry under 0 5/85ps impulse voltage. Gap length 27.5 mm Tip radius 30pm;
Crest voltage 178 kV (a) Still photograph (frame exposition 1ps), (b) Streak photograph, (c) Schlieren photograph (frame exposition 8 ns) [3]
14, No. 2
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7
) at the tips of bush-like negative primary streamers,
a-like structures occur. At higher frequencies, the
conversion from one polarity to the other requires a finite
time, I.e., new streamer patterns cannot occur until all
charge carriers of opposite polarity have been removed.
GAL
CURRENT
AND LIGHTEMISSION
ient current pulses are generally accompanied by
ssion pulses. The current signal is often measured
he medium of the voltage across a non-inductive
series with the test cell or by using optoelectronic
techniques. The light emitted by streamers is detected by a
ier tube or some other optical detector (e.g., a
ity that can exceed 100 kmls. For larger gaps ( > 4 0 mm), a
secondary stage could be detected. Depending on the electric field, the negative secondary streamer structure (NSS, or
. When the volt-
tip of the former positive streamer, which simultaneously
changes into a negative streamer.
During propagation, the streamer pattern changes due to
the alternate polarity of the consecutive half waves: (i)
straight branches of the umbrella-like positive primary
streamers disappear and several main branches are gener-
and the shape of the streamer current and
nding emitted light in liquid dielectrics depend
ers. For small gaps (<5 cm) these curreiits
more than a few mA) and their shape depends
ty of the sharp electrode. In a point-plane electhe current of a slow bushlike-negative
f bursts (< 1 0 ns), irregularly spaced, with
enerally increases with time throughout
er propagation. The light, which is emitted across
ap, has a similar shape to that of the current. The
number and amplitude of both streamer current and emitted
light pulses increase when the voltage is increased for a given
liquid [4-61.
ght emitted by the fast filamentary-positive
onsists of a unique pulse, the current having a dc
[4, 5-91 on which are superimposed other
ch more regular than with a negative streamer.
reases continuously to a maximum value gend to the time at which the streamer reaches
g sheet covering the opposite electrode.
For both streamer polarities, the amplitude of the current
and the intensity of the emitted light increase with the propagation velocity of the streamers. The currents of fast streamers are always higher than those of the slow ones, whatever
the polarity and the tested liquids.
For long gaps (5 to 100 cm), both positive and negative
currents consist of irregularly spaced pulses [lo].
d-plane geometry, these pulses can reach a few ama 100 cm gap. The current and emitted light wave0th negative and positive streamers under ac voltages
to those observed under iinpulse or step voltages.
the current, the spatial distribution and amplitude
ctrical charge in positive and negative streamers dee shape and velocity of the streamers themselves.
electrical charge of fast positive and negative
higher than that of the slow ones. Moreover, the
lectrode gap andlor the point radius andlor the
oltage, the lower the streamer velocity and the
lower the total charge. The more energetic the streamer, the
higher its velocity.
8
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Table I: Negative Streamer
cyclo-hexane (initiation)
PFPE:perfluoro-polyether Stl:steel
TF0:transformer oil
Cu:copper
vat, :average speed
SG:shadowgraph
LE:light emission
SS:single shot
1CC:Image Converter Camera
VCR:video recorder
March/April 1998 - Vol. 14, No. 2
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9
Table II: Positive Streamer
Liquids
I
n-pentane
n-hexane
n-heptane
n-octane
n-nonane
n-decane
dodecane
hexadecane
squalane
n-hexane (5 MPa)
n-hexane (+0.1% lML\T)
n-hexane (+5% DMA)
Icyclo-hexane
1&0(2-20.2MQ.cm)
IL-Helium
L-Nitrogen
1 1 :;
IWI
L-Nitrogen (initiation)
20pm
lpm
PFPE
Stl
35 pm
PFPE (initiation)
Si-Oil (100 cSt)
-TFO
w
w
w
1w
20pm
TFO (naphthenic)
Cu
06"
10pn
White-011 (n an h thenic)
Stl
3wm
1MN.l-methylnaphthalene W.tungsten
DPvU.N,N'-&methylamhne Stl:steel
NB:nitrobenzene
Cu:copper
TCE:tetra-chloroethylene
D0P:di-octylphthalate
AN.acetonitril
67mm
1
1'180p
1
25 4 m m (
I
I
112500
1
17.80 I 4 . 9 ~ 1 bush
+327 12100(l / 6 5 0 0 filament
I
+63 0
I
I
180 /
1071
tree
v,,,:maximum velocity
U,,
mean velocity
I
LF,
I
1
1 1
I
SChI1,
ss I
l9
22
Schn Schlieren
SG:shadowgraph
LE light emission
SS.single shot
1CC.Image Converter Camera
VCR vldeo camera
PFPE.perfluoro-polyether
TF0:transformer oil
10
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INFLUENCE
OF LIQUIDSTRUCTURE
The molecular structure has a significant effect on the
streamer propagation. The main parameter affecting
streamer propagation is the electronic affinity of the liquid
molecules. In halogenated liquids, the positive and negative
streamer velocities are very important compared with those
in liquids without halogens (Tables I and 11). The presence of
a single atom of chlorine in chlorocyclohexane leads to a
large increase in the negative streamer velocity, about ten
2
4
6
10
8
12
I
2
6
8
5
7
10
14
1
3
1
9
(4
4"
2
6
4
8
10
$4"
3
1
5
9
7
11
13
(4
2
6
4
8
10
1
12
I
3
1
5
7
9
5
7
9
11
4mn
11
13
7
9
I
14"
3
5
(b)
(b)
1
3
15
(c)
Fig. 2 Negative bush-like streamers in saturated hydrocarbon liquids. Gap length 4
mm; Tip radius 20pm. (a) n-pentane: - 45.9 kV, 2,us/Frame; (b) n-heptane: - 48.1 kV,
2 &Frame; (c) n-decane: - 50.4 kV; 2 &Frame [I 41.
Fig. 3 Streamer structures in benzene. Gap length 4 mm; Tip radius 20pm. (a) treelike: - 45.1 kV, 1 pdfraine. (b) bush-like: - 51.5 kV, 1 ps/Frame [14].
times that of cyclohexane [5]. When the point is positive, the
streamer is much more filamentary in chlorocyclohexane
than in other liquids such as cyclohexane, transdecahydronaphthalene and cis-decahydronaphthalene.
In most saturated hydrocarbon liquids and with larger tip
radii ( > 10 pm), the negative streamer has a bush-like structure (Fig. 2), whereas in pure aromatics (unsaturated hydrocarbon) liquids, a tree-like structure can also appear (Fig. 3 ) .
A negative filamentary streamer has been observed in liquid
helium and in liquid nitrogen. Figure 4 shows a positive
streamer having ,I radial structure in benzene.
Investigation of a number of silicone fluids of identical
chemical nature but with viscosity varying from 10 to
10,000 cSt found no significant change of streamer velocity
or shape [ll]. Similar results, including no significant
change in time to breakdown, have been found for polydimethylsiloxanes, also varying by a factor of 1000 in viscosity.
These results indiicate that viscosity has little effect on the
breakdown process.
Other work found similar expansion rates of shock waves
resulting from breakdown in liquids of differing viscosity,
such as n-hexane, isooctane, cyclohexane, and toluene [12].
It is difficult to find a simple relation between mass density
and the streamer velocity.
March/April 1998 - Vol. 1 4 , No. 2
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11
7
4
7
4
6
8
1
3
5
7
10
12
14
16 18
9
11
13
15 17
8
(a)
Fig 4 Positive radial streamer in benzene. Gap length 4 mm; Tip radius 20pm,
kV, 1 psiframe [I41
+ 50 4
2
4
1
3
5
7
9 1 1 1 3 1 5
(4
According to the value of this field, it could be possible to initiate different structures of streamers. Thus, if the radius of
curvature of the electrode point (rJ is sufficiently small, a
modest voltage can give a large electric field at the needle tip.
For impulse and ac voltages for ro < 3 pm, streamers iiiitiate
for E, > 10 MV/cm while for ro > 100 pm, streamers initiate
when E, > 1MV/cm. E, is the field determined by assuming
the electrode point to be a hyperboloid of revolution with no
injected charge.
The streamer structure can change with the radius of the
point electrode. For smaller tip radii (<1pm), the structure
of a negative streamer in cyclohexane changes from spheriherical (b), pagoda-like (c), and bush-like (d)
applied voltage as shown in Fig. 5.
In highly divergent electrode geometries under ac fields
with gaps -25 mm, with a mean field E <40 kV/cm, breakdown is controlled by the propagation of positive stream
whereas in moderately divergent geometries with gaps
mm and a mean field E< 80 lV/cm, breakdown is controlled
by negative streamers. The time to breakdown usually increases linearly with gap, while the amplitude, duration,
number, and length of partial discharge streamers decrease
with increasing gap.
2
2
4
12
4
16 18
2
-
A
v 100 ton
EFFECTS
OF HYDROSTATIC
PRESSURE
AND TEMPERATURE
As the hydrostatic pressure is increased, the electrical
breakdown strength generally increases, the number and
amplitude of current and light pulses are reduced, and the
streamer shape and length decrease, tending to form a string
of globules (Fig. 6) [13]. Above a threshold pressure that depends on streamer energy, the streamer, the corresponding
1
3
5
7
9
11
1 3 15 17 19
(dl
Fig 5 Various streamer structures in cyclohexane Gap length 5 mm, Tip radius 0.5
p m for (aj-(c), 1 5 p m for (d). (a) spherical: - 6.18 kV, 100 nsiframe; (bj hemispherical. -7 95 kV, 1 psikame; (6) pagoda-like, - 7 95 kV, 1 psiFrame, (dj bush-like 29 14 kV, 100 ns/Frame [14].
12
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1
1.0 bar
1.1
1.3
I
1.5
Fig. 6 Influence of hydrostatic pressure on the shape of the negative streamer in cyclohexane. Voltage 28 kV; Gap length 1 mm; Tip radius 5pm. All photographs are taken
after 7 p s [18].
c
Wmin (kmk)
EFFECTS
OF AtlDlTlVES
0.7
I
0
0.04
0.08
I
0.12
0.16
0.20
Fig. 7 Influence of C C 4 concentration on the negative streamer velocities in cyclohexane under step voltage. Gap length 2 mm; Tip radius 3 pin; Voltage 33 kV [13].
1.0
’
6.0
-
5.0
-
I
I
I
I
Fig. 8 Effect of DMA concentration on the positive streamer velocity in Marcol 70 and
2,2,4-trimethylpentane [6].
March/April 1998 -Vol.
current, and light pulses disappear [ l l , 141. The higher the amplitude and the number of the current (and the light) pulses, the
higher is the hydrostatic pressure necessary to cause vanishing
of the current pulses. No appreciable effect has been reported
when reducing the pressure below atmosphere.
At atmospheric pressure, temperature has only minor effects on streamer behavior. The number and amplitude of
current and light pulses from slow streamers increase with
temperature while filamentary streamers are unaffected by
temperature.
Small concentrations of polyaromatic compounds greatly
reduce the impulse breakdown voltage of a naphthenic oil
between a negative point and a grounded sphere [15]. This
reduction in the breakdown voltage is due either to an increase of the streamer velocity or to the diminution of the
initiation volta,ge. Since these compounds have both low
ionization potentials and large electronic-trapping sections,
it is difficult to distinguish the phase (initiation or propagation) that is affected by their presence. In pioneering work,
Devins et al. [11] separately studied the influence of each
property of additives (electronic scavenger or low ionization
potential) and showed the importance of electronic processes. They observed that the addition of a non-ionic electronic scavenger such as sulfur hexafluoride (SF,) or ethyl
chloride (C22H5Cl), to a naphthenic oil or to 2,2,4trimethylpentane renders the negative streamers more
filamentary and increases their velocities [4]: With 0.05
mol/l of SF, or C2H,C1, the streamer velocity can reach five
times its initial value. There are no detectable effects on the
positive streamers in these liquids. These results have been
confirmed by others. By adding 0.04 mol/l of CC1, to cyclohexane, Beroual et al. [5] observed that the negative
streamer velocities increase by a factor of 10. Above 0.04
molil, the increase of the velocity becomes less important
(Fig. 7 ) .There are no detectable additive effects on the positive streamers iin these liquids.
The addition of a non-ionic low ionization potential compound such as N,N-dimethylaniline (DMA) does not change
the negative streamer velocity, whereas it increases (by a factor of two to three) the positive streamer velocity in a
naphthenic oil (Marcol 70) and in 2,2,4-trimethylpentane
(Fig. 8). In both cases, there is a concentration (about 0.05
mol/l) above which no significant increase in the velocity is
observed [4].
The addition of a low ionization potential compound
(0.05 mol/l) !such as tetramethyl paradiphenylamine
(TMPD) (Fig. 9 ) to cyclohexane leads to a moderate increase
of the negative streamer velocity (by a factor lower than two)
and the shape of the streamer is practically unmodified [ll].
The positive streamer velocity, under the same conditions, is
multiplied by a factor of three and they become still more
filamentary. A similar effect is observed with DMA [4, 16,
171. The influence of electronic scavenger and low ionization potential c,ompounds is comparable in other liquids,
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13
MiDBT is a blend of two isomers
(mono(C,,H,,)
and di(C,,H,o) benzyltoluene)
CH
\3
@ ) - C H 2 a
U
mono
U
such as phenylxylylethane (PXE) and mono-dibenzyltoluene
(M/DBT) [14], a blend of two isomers (mono(C,,H,,) and
di(C,,H,,) benzyltoluene) (Fig. 9).
Ionic additives such as picrate of triisomylammoniuin
(TIAP) and Aerosol OT (Fig. 9) also have an influence on the
streamer shape and velocity [ll].With a concentration of
AOT comparable to that of CCl, in cyclohexane (typically
about 10 mol/l), the velocity when the point is negative is
multiplied by a factor of ten as in the case of CCl,.
Additives also change the shape and amplitude of currents
for slow negative streamers. The number and amplitude of
the current peaks increase when adding electronic scavenger
additives such as CCl, [14]. A similar effect is observed in
other liquids such as M/DBT and PXE.
SPECTRAL
AND CHROMATOGRAPHIC
ANALYSES
PXE (phenylxylylethane (C,eH,8))
TiAP (triisomylammonium picrate)
1
H,,c~-
C5H11
NH+O- @NO2
I
1
NO*
Aerosol OT
(sodium di-(2-ethylhexyl) sulphoccinate)
CH
,,l
-0-C
No
‘CH2
I
CHS03C,H,,-O-C’
Na+
‘b
TMPD
(tetramethyl paradiphenylamine ((CH,),(C,H,),NH))
Spectroscopic studies of the light emitted during the prebreakdown phase in liquid dielectrics extend from the UV to
the visible range [18-201. They revealed, in n-hexane, the
presence of atomic and molecular hydrogen and carbon
molecules (C, and CJ, and tiny amounts of metal emanating
from the electrode metal. The formation of these species is
attributed to an electron avalanche mechanism similar to
that known to occur in gas discharges.
The spectra obtained in liquids such as cyclohexane
(C6H1,), a blend of mono- and dibenzyl toluene (M/DBT),
and phenylxylylethane (PXE) (Fig. 6), in a point-plane electrode system, reveal spectra lines (emerging from a continuum corresponding to molecular fragments) belonging to
atomic as well as molecular hydrogen (H, line) and carbonaceous matter (C, Swan bands) resulting from the decomposition of the fluid (Figs. 10 and 11)[20].These characteristic
lines are more intense when the point is an anode than when
it is a cathode. This parallels the fact that positive streamers
are much more energetic than the negative ones.
The more energetic the streamer, that is, the faster it is,
the more important the characteristic lines (Hrand Ha lines,
and C, Swan bands in this study) emerging from the basic envelope of the light emission spectrum. This indicates that the
higher the streamer energy, the higher the possibility of the
dissociation of the liquid molecules.
Results of chromatographic analysis of the dissolved gases
generated by streamers in liquids such as PXE and M/DBT
confirm those obtained with the spectroscopic analysis. The
products revealed by chromatography are H,, CH,, C2H4,
C,H, and C,H,. Hydrogen and acetylene are mainly present
and their quantities are much higher in positive polarity than
in negative polarity for a given liquid [20].These products
are likely the results of the recombination of carbon and hydrogen resulting from the dissociation and fragmentation of
liquid molecules.
Models and Discussion
Ck,
Fig. 9 Structure of oil components
The correlation observed between the shape and the velocity of the streamer, the corresponding current, and the associated light emission suggested an estimation method of
14
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10001
1
500
2 400
2o01
100
"
516
514
512
510
508
506
504
664
502 nrr
662
660
658
656
654
652
650
nm
(a)
b
600
2
400
0
550
500
450
5
400
0
,- 350
5C 300
2 250
200
150
"
300
200
.
"
I
I
'
I
'
9 "'
L
50
loo
664
662
1
660
'I
658
""
516
514
512
510
508
506
504
656
'
654
'
'I
652
650
I
nm
(b)
502nn
Fig. 10 Streamer spectra in cyclohexane under step voltage: Voltage V = 32 kV; Gap
length L = 0.5 mm ;Tip radius rp = 3pm. Central window of the spectrograph (dispersion 1.2 nm/mm) set at 509.3 nm: (a) fast positive streamers, (b) fast negative
streamers. Accumulated data of about 100 streamers [23].
Fig. 11 Streamer spectra in cyclohexane under step voltage: Voltage V = 32 kV ;
Gap length L = 0.5 mim ;Tip radius rp = 3pm. Central window of the spectrograph
(dispersion 1.2 nm/mm) set at 656.3 nm: (a) positive streamers, (b) negative
streamers [23].
the current using simple models [21]. The slow bush-like
streamer is assumed to be a conducting sphere growing from
the point towards the plane, and the fast filamentary
streamer is modeled as an extension of the point moving towards the opposite electrode. These models have been proposed elsewhere [5, 22, 231 to correlate the electric field to
the streamer velocity. However, even though assuming the
streamer as a conductor found some confirmation using Kerr
effect measurements in nitrobenzene, one must be very careful when considering the shape of fast or slow streamers,
since neither the shape nor the order of magnitude of the
measured currents corresponds to either of these models.
Moreover, currents corresponding to filamentary streamers
are much higher than those of bushlike streamers, unlike
those predicted by the above models. Consequently, these
oversimplified physical representations are not very helpful
in correlating the transient current and the velocity. It could
be interesting to search other models and methods to evaluate the transient currents.
To explain the viscosity effect of liquids, Watson et al. [22]
proposed a model in which the viscous drag is balanced by
the electrostatic force at the streamer tip. This model has a
viscosity limiting the growth of an electrohydrodynamic in-
stability driven by the electrical force on the streamer charge.
This causes an initially spherical cavity to form a multibranched structure. In supporting experiments [24], low viscosity liquids produce bushy negative streamers with numerous branches, while high viscosity liquids produce
spheroidal shadows with few or no branches. The shape and
growth rate of the experimentally observed streamers, in liquids of low and high viscosity, seem to be in a good accordance with the electrohydrodynamic instability model.
According to other investigators [ l l ] , the model proposed by Watson et al. [22] is somewhat in contradiction
with the experimental results concerning the influence of the
viscosity. Indee'd, by varying the viscosity by three orders of
magnitude (from 90 to 4 ~ 1 cSt
0 ~at 20' C), no significant
variation of the. streamer velocites and their shape was observed. Therefore, the influence of the viscosity must be further investigatf d with additional experiments on different
liquids.
In order to explain the influence of additives and the molecular structure of the liquid medium on streamer propagation, Devins eit al. [4] proposed a model in which they
assumed that field ionization occurs in the liquid. They used
Zener's theory of tunneling in solids to calculate the concen-
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low ionization potential compounds increases the rate of
ionization and reduces the time t, spent in the second step,
and thus also increases the velocity. Further research needs
to evaluate the times t, and t,.
In recent work, Beroual [14]has proposed a model explaining the relationship of the current and the corresponding electrical charge, the emitted light, the shape, and the
velocity of streamers. This model, based on energetic considerations, explains the pre-breakdown processes that can occur, the propagation mode of streamers (i.e., by steps or
continuous), and the influence of additives, and it evaluates
the propagation velocity of the streamer. It is shown that the
streamer that consists of irregularly spaced current bursts
moves by jumps. The higher the frequency and amplitude of
the current pulses, the shorter the duration between these
steps and the higher the average velocity of the streamer.
Then, the streamer tends to propagate continuously. This
can be obtained by adding an electronic scavenger (to a slow
negative streamer) or low ionization potential (to a fast positive streamer) compounds or yet by increasing the voltage
(Fig. 12).The streamer velocity can exceed 100 km/s. On the
other hand, the higher the elementary charge corresponding
to each current pulse, then the more energetic the streamer,
the higher will be its velocity and the more filamentary will
be its shape. This model must be further completed by estimation of the local electrical charge at the streamer head
during its propagation.
(b)
Fig 1 2 Structures of positive streamers in n-pentane Gap length 4 mm, Tip ra
dius 2 0 p m (a) tree-like + 40 4 kV, 100 nsiframe, (b) bush-like + 30 9 kV,
500 ns/Frame [I41
tration of the positive and negative carriers contained in a cylindrical conducting channel. The propagation of negative
streamers occurs in two stages: electron injection and trapping followed by ionization within the liquid. It results in a
plasma similar to that produced when the point is positive.
This model does not provide an evaluation of the time spent
in each mode.
On the other hand, according to Devins et al. [4], the
streamer velocity U increases when the mass density p decreases. This is in contradiction with the experimental results. Indeed, by considering two dielectric liquids having
roughly the same mass density such as benzene (p=0.8879
g/cm') and chlorobenzene ( p = 1.107 g/cm3), it has been observed that the positive streamer velocities U can be different:
1.2 and 76 km/s, respectively, in the same experimental conditions [19]. It seems difficult to find a simple relation between p and U. On the other hand, the model proposed by
Devins et al. states that the velocity of positive streamers is
constant, whereas it is not correct as shown experimentally
by others [5, 231. In spite of the contradictions discussed
above, the model of Devins et al. is supported by two facts:
(I) the addition of electronic scavengers reduces the trapping
distance and the time t, spent in the first step, and thus increases the negative streamer velocity; (11) the addition of
CONCLUSIONS
The streamer structure and its characteristics (velocity,
current and emitted light, conductivity. ..) depend on many
parameters, especially the chemical composition of the liquid (pure or containing selective additives); the applied voltage (shape, magnitude, and polarity); the electrode
arrangement (gap length and electrode radius of curvature);
and the hydrostatic pressure. Depending on the experimental conditions, different structures and propagation modes
of streamers can be observed. The main parameters governing the streamer structure through the electrode gap (i.e., at
each step of the streamer development) are the local electric
field on the streamer tip, the average electric field in the gap,
and the physico-chemical properties of the liquid.
Streamer propagation phenomena in dielectric liquids are
governed by both electronic and gaseous mechanisms. The
former dominates when the energy injected in the medium is
very important and/or in presence of halogens or aromatic
molecules in the liquid. This is supported by the following
facts: (i) the strong influence of small concentrations of electronic scavenger or low ionization potential additives on the
shape and velocity of streamers, the corresponding current
and emitted light, and the spectroscopic analysis of the emitted light by streamers, which indicate the presence of electronic processes; (ii) the strong effect of the hydrostatic
pressure on the initiation and propagation of streamers, and
the corresponding currents and emitted light, which indicate
that the physical nature of the streamers is gaseous, con-
16
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firmed by chromatographic analysis of the dissolved gases
due to discharges.
Continuing investigations should be oriented towards
measurement of the conductivity of the streamers to expand
on results reported and discussed in the literature. The estimation of the temperature and the pressure within the
streamer may help to identify the nature of streamers. Systematic spectroscopic and chromatographic analyses in real
time can give more information concerning the physicochemical processes (ionization, dissociation, vaporization ...)
involved in streamer propagation.
Members of the Liquid Dielectrics Committee International Study Group who participated in this study and their
affiliations are: A. Beroual, Ecole Centrale de Lyon,
CEGELY-CNRS, Ecully, France; M. Zahn, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; A.
Badent, K. Kist, and A.J. Schwabe, the University of Karlsruhe, Germany; H. Yamashita and K. Yamazawa, Keio University, Yokohama, Japan; M. Danikas, Democritus
University of Thrace, Xanthi, Greece; WG. Chadband, Salford University, Salford, UK; and Y Torshin, All-Russian
Electrotechnical Institute, Moscow, Russia. The corresponding author, A. Beroual, may be reached at CEGELY - Ecole
Centrale de lyon, B.P. 163, 69131 Ecully Cedex, France.
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14, NO.2
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